Add VERSION file as the single source for the extension version

Mirrors testium's src/VERSION: scripts/sync-version.js copies VERSION into
package.json#version (enforcing strict semver X.Y.Z, required by the marketplace
/ Open VSX). Wired into 'npm run package' and vscode:prepublish, so the version
is always synced before a build/publish. Bump = edit VERSION.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.vscodeignore

@@ -11,3 +11,5 @@ node_modules/.cache/**
 .github/**
 .editorconfig
 icon.svg
+scripts/**
+VERSION